In wideband receivers such as those used for wireless communications, the entire RF (radio frequency) band allocated for cellular operations is digitized with a high fidelity ADC (analog-to-digital converter). In a basestation wideband receiver capable of simultaneously processing CDMA (code division multiple access), TDMA (time division multiple access) and GSM (groupe special mobile) signals, digital channels with a respective 1.23 MHz, 30 KHz and 200 KHz bandwidth are extracted from the digitized signal produced by the ADC.
However, the RF signal received by the wideband receiver may experience rapid and wide variations in signal power due to interferences from unrelated signals such as signals emitted by mobiles associated with competing service providers. Typically, the dynamic range of the wideband receiver is limited and therefore not suited to accommodate large fluctuations of the signal input power. In particular, if the overall gain and noise figure of the receiver chain are designed such that the lowest signal level received at the antenna can be processed to meet the RF sensitivity requirements, the highest signal level may overload the ADC and cause clipping of the digitized signal produced therein. This would severely impact the wideband receiver performance for other cellular users in the cell, particularly those with a low signal level since clipping of the digitized signal would result in corruption of the data extracted therefrom.
A wideband receiver will generally include an AGC (automatic gain control) circuit for counteracting overloading of the ADC due to high received signal strengths and attenuate the ADC input power within the ADC dynamic range. The AGC circuits currently available present various attenuation responses which are characterised by an attack time defined as the time required for eliminating clipping of the ADC output signal. Unfortunately however, these AGC circuits exhibit a relatively slow attack time and are therefore not designed to effectively minimize the ADC clip time. For optimal cellular communications, it would be desirable to reduce the ADC clip time so as to improve the overall performance of the wideband receiver.
Additionally, strong ADC input signals can be attenuated to fall within the ADC dynamic range with a large AGC range. In particular, it would also be desirable to have an AGC designed with a large range for accommodating large fluctuations in the power of the ADC input signal.
Accordingly, there is a need for a AGC circuit which has a large range for accommodating large fluctuations of the ADC input signal power and a fast attack time for improving the performance of the wideband receiver and more particularly for reducing the ADC clipping time to a level appropriate for cellular communications.